The present invention relates to a method of making a subframe of a motor vehicle, and to a subframe for a motor vehicle.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Contemporary motor vehicles have subframes to receive components of the chassis and/or drive unit of the motor vehicle. Subframes are thus also part of the vehicle axle and stiffen the front structure and/or the rear structure. Therefore, subframes are firmly secured to the body of the passenger vehicle.
Stiffness of a subframe is crucial because it is subject to high static and dynamic forces during normal travel. As a result, subframes are complex components which must meet increasingly more stringent standards. On one hand, the deformation behavior must be optimized in the event of a crash, and yet the subframe should be lightweight and at the same time rigid and torsionally stiff.
To address the complexity issue of the geometry in the area of the attachment points, cast nodes of aluminum have been provided in the area of the axle and connected via profile elements made in particular as extruded intermediate pieces. This approach is very expensive in view of the exclusive use of aluminum. Moreover, the need for a welding joint to connect the cast nodes with the extruded profile is problematic in view of the different alloy composition and the substantial difference in the geometry. As hollow and thus lightweight castings are difficult to make and thus expensive, ribbed open structures are used. Such ribbed structures exhibit, however, inadequate torsion stiffness when compared to closed steel pipe or shell structures, especially when correlating the torsion stiffness to the mass of the profile element.
Other shortcomings of aluminum include the small modulus of elasticity of aluminum which translates to a reduced stiffness in particular when small-volume structures are involved. The size of a structure cannot fall below a certain minimum level, despite the tight space available. To still realize the necessary stiffness, extruded profiles would have to be made thicker. This, however, is not an economically viable solution. On the other hand, the outer dimensions of the profile elements cannot be increased in view of space limitations. Ribbed cast nodes thus are inadequate to have the desired torsion stiffness.
It would therefore be desirable and advantageous to address this problem and to obviate other prior art shortcomings.